The present invention relates to the optical constitution of an optical head in an optical information processing unit for recording, reproducing, erasing the information signals such as data, image, sound, etc.
In order to obtain the focusing error signal and the tracking error singnal from the optical head, various methods have been made public using a beam splitter, a cylindrical lens, etc. An example of the methods utilizing such technique is described in Japanese Patent Laid-Open No. 2-193344, the operating principle of which is explained hereunder in its outline.
An example of prior art constitution is illustrated in FIG. 7(a), FIG. 7(b) and FIG. 7(c); where, FIG. 7(b) and FIG. 7(c) are the views as seen in the direction A and direction B, respectively, indicated in FIG. 1(a). The light emitted from a semiconductor laser 101, being an example of light source, goes through a collimating lens 102, a beam splitter 103, an object lens 104, to be condensed on an optical disk 105. The light reflected by optical disk 105 is reflected by beam splitter 103, and goes to a beam splitter 108 through a wave plate 106 and a single lens 107. The light reflected by beam splitter 108 is separated into light 111 and light 112, each of which goes to a photosensor 110 through a beam splitter 109. Light 111 goes to a detector 113 on photosensor 110 through beam splitter 109. Light 112 is separated into light 112a and light 112b at beam splitter 109, and both lights go to a detector 114 on photosensor 110.
The focusing error signal is detected by the SSD(Spot Size Detection) method, the operating principle of which method is described in detail in the specification of Japanese Patent Laid-Open No. 2-192031. Outline of the method is that the focusing is obtained by controlling the spot sizes of light 112a and light 112b, which takes a light path different from that of light 112a, on detector 114 to be equal.
The tracking error signal is detected by the Far-Field method, the operating principle of which method is described in detail in U.S. Pat. No. 4,491,940. Outline of the method is that the tracking is controlled by detecting the intensity variation caused by interference of the zero order beam reflected by optical disk 105 and the +1 order beam diffracted by optical disk 105.
The above described prior art constitution has drawbacks that the cost of manufacturing an optical head inevitably goes high and the designing of a compact, light-weight and simple-structured optical head is blocked because of the use of an expensive, and bulky, beam splitter and other optical elements as the constituent component.